Film drive system



March 8, 1949. G. A. MITCHELL FILM DRIVE SYSTEM 2 Sheets-Sheet l Filed Sept. 4, 1945 y INVEN GEORGE A. MITCHELL BYE ATTORI;

March 8,v 1949. G, A, M11-@HELL 2,463,548

FILM DRIVE SYSTEM lNVENTOR SEORGE A. MITCHELL ATTORNEYS Patented Mar. 8, 1949 yUNITED STATES PATENT OFFICE FILM DRIVE SYSTEM George A. Mitchell, Pasadena, Calif., assignor to Mitchell Camera Corporation, West Hollywood, Calif., a corporation of Delaware Application september 4, 1945, serial Ne. 614,281

(ci. ss-i'z which progressively varies as the roll of film on' the spool increases in diameter. In speaking here of uniform speed, that is meant with relation to the speed of the driving mechanism from which both the film movement and the take-up device are driven.

In motion picture cameras and projectors, and in sound recorders and reproducers, a movement mechanism of one kind or another drives the film past an exposure aperture at an average uniform speed. In a typical instance, the film is taken up from the feed sprocket of the movement by -a take-up reel or spool on which the iilm is wound in a coil of constantly increasing diameter. The take-up spool must be driven in such manner as to maintain tension on the film and to compensate for the varying diameter of the coil. Although various other expedients have been proposed, the commonly adopted drive for the takeup spool is one involving frictional slippagefor example, a belt drive maintained under suitable tension to slip at the desired film tension.

Any such frictionally slipping drive is objectionable on at least two accounts; it 'is subject to continuous wear and possible mal-functioning or complete break-down, and it consumes an amount of power which is not an inconsiderable fraction of the total amount required to drive, for example, a motion picture camera. Driving motors for such cameras must be larger and heavier than would otherwise be necessary; and that is a, matter of considerable importance particularly in hand cameras.

The general purpose and objective of the present invention is the elimination of frictional slippage and power losses in the drive of take-up devices. I have discovered that, with proper proportioning with regard to relative speeds, gear ratios and film tensions, the film movement and the take-up can be driven from two driven members of a differential gearing, the driving member of which is driven from any suitable power source, preferably one of constant speed. With the movement and take-up driven in that manner from a constant speed source, the movement speed necessarily varies as the rotational speed of the take-up yvaries with increasing diameter of the film coil; but I find that, with proper proportioning of the driving mechanism, the film may be kept under proper take-up tension and the movement speed may be maintained constant within a very small percentage even though the flhn coil diameter varies by a factor of four or more.

The accompanying drawings illustrate a typical application of my invention to a motion picture camera. In those drawings:

Fig. 1 is a plan-section showing an illustrative and typical form of my drive mechanism;

Fig. 2 is an elevation of the parts shown in Fig. 1; and

Fig. 3 is -a schematic elevation in the same aspect as that of Fig. 2, illustrating the drive mechanism in modified form.

In the drawings a typical mounting plate or bracket is shown at I0, to carry to various parts of the mechanism. The numeral Il designates an intermittent film driving mechanism or movement which drives the film past an aperture (not shown) in aperture plate I2. The intermittent movement needs no detailed description, as it can be of any type and kind and, in fact, can be'a continuous nlm driving movement. Whatever its type and character, it will be assumed merely for the purpose of this description that the film movement is driven from movement shaft I3 by gearing I4 in such ratio that the lm is moved past the exposure axis through the distance of one frame for each rotation of shaft I3. Shaft I3 as here shown is also the shutter shaft carrying the rotating disk shutter I3a.

The iilm moving mechanism as here shown also includes the large film feeding sprocket I5 which is driven by gearing I6 from shaft .I3 at l such a ratio that the lm, held in engagement with the upper side of the sprocket by rollers I'I, is fed into the upper film loop and toward the intermittent movement at a rate of one frame for each revolution of shaft I3, and the film also held in engagement with the lower side of the sprocket by rollers I8 is fed away from the lower loop lat the same rate. The lm is thus fed to and through the intermittent movement and away from it, at the rate of one frame for each revolution'of shaft I3. That particular relation of film movement and shaft rotation is of course not necessary; but it is a common relationship and is here adopted for simplicity of description and discussion.

The initial or primary drive shaft for the mechanism is shown at 20 and may be driven by any suitable power source, preferably at a .constant speed. A synchronous motor drive may be taken as typical. The film take-up shaft, or the shaft which may drive the film take-up through gearing, is shown at 2|. In Fig. 1 the end of shaft 2l is shown equipped with a bushing 2la to take the film spool or reel directly; and in some instances that may be the arrangement. On the other hand, as shown in diagram in Fig. 3, shaft 2| is connected by the gearing train 22, 23, 24 with shaft 2lb which carries the film take-up spool 29. In either case shaft 2l is typically a film take-up drive shaft. It is mounted in bearings lila in frame bracket l0. Gear 22 of Fig. 3 may be mounted on the end of shaft 2l in place of the bushing 2 la which is removed.

Primary drive shaft drives both the movement shaft I3 and take-up drive shaft 2l through the intermediary of a differential gear mechanism. That differential mechanism may be of any of various known types. For instance it may be of the type commonly used in automobiles where the two driven gears are ring gears of equalA diameter, and the bevel-gear planetary or planetaries operate between the opposed toothed faces of the two ring gears. However, I prefer to utilize a differential mechanism of the type employing an external ring gear, and internal sun gear, with an intervening -planetary or planetaries which are in the form of spur gears; and I show such a type of differential mechanism in the drawings.

, The internally toothed ring gear is shown at 25, carried on a disk or web 25 which is journaled freely on shaft 2l to rotate relative to that shaft. This ring gear, in the typical design here shown, drives the film movement, and although it may drive the :nlm movement directly or, what amounts to the same thing, through an even ratio gear connection, I here show the ring gear as driving movement shaft i3 through a gearing connection having a ratio of three-to-one. Ring gear web 26 carries the bevel gear 2li which meshes with the smaller bevel pinion 2t on movement shaft i3.

The sun gear of the differential is shown at fill directly on take-up drive shaft 2l. Two planetaries 3l are shown rotatably mounted on studs 32 which are carried by the web of a bevel gear 34 which meshes with a bevel pinion 35 on initial drive shaft 2G. Gear web 33 is journaled on shaft 2i to rotate freely about the -shaft axis. The planetary gear mechanism is thus driven through geai` 34 and its web i3 which acts as the rotary carrier of the axes of the two planetary gears. That is, the planetary mechanism is driven by driving the carrier of the planetary axes, and the planetary gears in turn drive both the ring gear and the central sun. gear 3d, the ring gear driving the film movement and the sun gear driving the take-up drive shaft 2l.

Fig. 3 shows in diagram all of the essential parts of the system, designated by numerals corresponding to those used in Figs. l and 2. Film F is shown threaded over feed sprocket l5, through movement i l, under sprocket l5 and then going to the take-up spool 29. As stated before, spool 29 is here shown as driven through gear train 22, 2.1i, 2li from sun gear shaft 2l. The gearing ratio shown in Fig. 3 between shaft 2f and spool 29 is three-to-one (in gear sizes) looking from shaft 2l to the take-up. For diagrammatic and descriptive purposes, a carrier A for the planetaries is shown in the form of an arm. This carrier A corresponds to the power driven gear 3f! and its web 33 of Figs. l and 2. `While drive shaft 2li of Fig. l has been spoken of as the initial drive shaft for the mechanism, its function is merely to drive carrier A through gears 34, 35. Carrier A is therefore the real initial driving member of the differential train and of the whole drive mechanism.

If we assume that carrier A is rotated clockwise in Fig. 3, then, with sun gear 30 stationary, ring gear 25 is also rotated clockwise at an angular speed determined by R-l-S R where R and S denote the pitch diameters or numbers of teeth of the ring and sun gears. As shown in the drawings, ring gear 25 has 48 teeth and sun gear 30 has 16 teeth. With the sun gear stationary, the ring gear is driven at $/3 the angular velocity of carrier A, and, with the three to one gearing 21, 28 driving the movement shaft I3 from the ring gear, the latters angular'velocity is four times that of the initially driven, carrier A. Assuming that under these conditions it is desired to drive the movement shaft at 960 R. P. M. (16 frames per second) then carrier A is driven at 240 R. P. M. The drive gearing 34, 35 shown in Figs. 1 and 2 has a ratio of one-to-four, so that initial drive shaft 2@ is in this instance driven at 960 R. P. IVI.

With the ring gear driven clockwise in Fig. 3, the reaction from the torque necessary to drive movement ll, transmitted from the ring gear through the planetary to sun gear S, tends to rotate the latter clockwise. With the sun gear Icoupled with the take-up spool 29 in the manner illustrated in the diagram of Fig. 3, the reaction torque also tends to rotate the take-up spool clockwise and thus tends to keep the film under tension and to wind the nlm on the spool as fast as the lm is fed forwardly to the spool by feed sprocket l5.

When the spool is empty, at the beginning of a take-up and winding operation, the effective spool diameter is a minimum and the nlm moves to the spool somewhat as indicated at F1 in Fig. 3. The particular spool here shown by way of typical illustration has a diameter of about 11%@ and at the end of the take-up and winding operation the diameter of the film coilthe effective diameter of the spoolis about 5". The nlm is indicated at F2 going to the spool at that maximum diameter. The minimum circumference of the take-up spool is approximately equal to lo frames of the nlm which is here being used as an illustration (16 mm. film) and the maximum effective circumference of the taire-up spool is equal to about lm frames.

Due to the fact that sun gear il@ is rotated forwardly to take up the film, the rotational speed of movement shaft i3 and the speed of the movement is at no time as high as it would be with the sun gear stationary and with the initial drive operating at the same speed. When the take-up spool is full, at its maximum effective diameter, the spool is of course rotated only V130 of its circumference for each rotation of movement shaft it. In the particular design and arrangement here shown, that means that the sun gear rotates forwardly only f/ggo of a complete revolution for each revolution of movement shaft it, and 1/130 of a revolution for each complete revolution of ring gear 25. Calculations and tests have shown that such slight relative lforward movement of the sun gear, when the film coil is at maximum diameter, is so small that the drop in speed of. movement shaft i3 below the normal Rev.g= (RevUi) selected speed of 960 R. P. M. is quite completely negligible (it is about 2 R. P. M. or a. drop in speed of about 0.002). For the purpose of ascertaining the overall variation in the speed of the movement shaft and movement, their speeds at the end of the winding operation may be considered as being the standard selected speed, and the small error resulting from that assumption makes the calculated variation in speed greater than that which actually occurs, rather than less.

The over-all variation in movement speed is ascertained by calculating that speed on the a-ssumption that the sun gear is rotating forwardly at the rate of approximately Il; of a revolution for each revolution of the ring gear. In the arrangement shown in Fig. 3, at the beginning of the take-up operation, spool 29 is rotating forwardly le of a revolution for each revolution of movement shaft I3. Sun gear 30 rotates forwardly at one-third the speed of the take-up, and ring gear 25 rotates at one-third the speed of the movement shaft; so that the sun gear rotates forwardly 11g revolution for each revolution of the ring gear. The calculation indicates that the movement shaft, at the beginning of the operation, rotates at about 940 R. P. M. instead of the standard 960. The relative drop in speed, in this particular illustrative ease, and checked by actual operation of the mechanism, is thus about 2%. That overall variation in speed is much smaller than the variation commonly resulting from the use of spring motor drives for hand cameras; and of course is also much smaller, and much more uniform, than the variation in any hand driven camera.

Formula 1 assumes the sun gear to be stationary and thus gives only that component of the ring gear speed resulting directly from rotation of carrier A. When the sun gear also rotates, the ring gear has another component of rotational speed, say

where the minus sign indicates that clockwise rotation of the sun gear produces counter-clockwise rotation of the ring gear.

The actual rotational speed of the ring gear is then Rev. 1R= (Rev. s)

as the value of Reva. The movement speed, three times that value, works out at approximately 940 R. P. M.

The tension maintained on the film by the takeup is determined by the torque which is necessary to drive movement shaft I 3, and by the overall gearing ratio between the movement shaft and take-up spool 29. The reaction torque is transmitted through the gear train; and in the particular illustration here given the overall gearing ration between shaft I3 and take up spool 29 is a ratio of three-to-one (stating the ratio in relative gear sizes and looking at the train from shaft I3 to the take-up). Thus, in the illustration given, the torque, in inch-ounces on the take-up spool is one-third that on the movement shaft I3. The absolute torque in inch-ounces, depends upon the torque necessary to operate the movement and other parts connected with shaft I3. In an actual structure, using a standard film movement and using the gear ratios illustrated, the torque on the take-up spool has been found to be suiiicient to insure film take-up at the full spool condition.

Many variations may be made in the design, within the limits of gearing ratios which impose acceptable tensions on the film, and within desirable limits for maximum overall speed variation in the movement shaft. As a simple illustration of one variation, ring gear 25 may drive the shutter shaft directly (or, what amounts to the same thing, with a gearing ratio at 21, 28 of oneto-one) and the sun gear shaft 2| may drive takeup spool 29 directly or at a one-to-one gearing ratio. In that case the resulting functioning as regards speed variation is the same as in the illustration given in Fig. 3; and the tension on the nlm is the same because the gearing ratio through which the reaction torque is transmitted from the movement shaft to the take-up, is still a ratio of three-to-one.

Variations in the relative size of the sun gear, everything else being the same, results in a change in both the over-all speed variation and the lm tension. For example, still assuming that the movement and take-up aredriven directly from the ring gear and sun gear, an increase in the sun gear diameter results in a correpsonding increase in the overall speed variation, the latter being about 3% if the sun gear is increased to half the diameter of the ring gear. And the torque transmitting gear ratio from the shutter shaft to the take-up is changed to two-to-one; that ratio is relatively reduced so the tension on the film is correspondingly increased.

If, in any given arrangement as to relative sun gear diameter and gearing ratio between it and the take-up, the gearing ratio (ratio of gearing sizes) between the movement shaft and the ring gear is decreased, the overall speed variation and the tension on the nlm are both increased. For example, with the sun gear one-third the diameter of the ring gear, the sun gear, driving the take-up directly, and the movement shaftA geared to the ring gear with one-to-three gearing (same as in Fig. 3 except that the sun gear drives the take-up directly or at one-to-one ratio) the torque on the take-up is increased by a relative factor of three, compared to Fig. 3 (the torque gear ratio is now one-to-one); and the over-all speed variation is multiplied by three. That is because, with thev movement running relatively three times as fast as the ring gear, the sun gear is allowed to move forward relatively three times as fast. But if now, in such a situation, we drive the take-up from the sun gear at a ratio of threeto-one (as in Fig. 3) the relative forward rotation of the sun gear is now reduced by one-third, to be the same as in Fig. 3 or as in the case where both movement and take-up are driven directly. An the over-all gear ratio between the movement shaft and take-up is again three-toone so that the lm tention is decreased to the same figure as when the movement shaft and take-up are driven directly.

To summarize generally, we may consider the effect of the various gear ratios (ratio of gear sizes) as they are looked at from the movement shaft to the take-up.

'Ihe over-all speed variation of the movement gnam ` gear'because the ring gear moves more slowly.

Decreasing the gear size ratio between the sun gear and the take-up has a direct effect of allowing the sun gear to advance more rapidly. Correspondingly, increase in the gear size ratio at any point in the train, looked at from the movement shaft to the take-up, has the effect of decreasing the relative advance of the sun gear and decreasing the over-all speed variation.

Consequently, increase in the over-all gear size ratio from the movement to the take-up, regardless of the point or points at which the increase .is made, decreases the take-up torque and the lm tension and also decreases the over-all speed variation at which the movement is driven. Accordingly, the optimum design for any given situation m-ay be said to be reached when the over-all gear size ratio from movement to take-up is as high as it may be, consistent with a sumcient pull on the lm at maximum take-up diameter, to insure the lm being taken lup and wound suiiciently tightly. The over-all speed variation is then the minimum.

In reaching the optimum design, involving minimum speed variation and minimum film tension, the fact that movement shaft i3 is subject to considerable inertia is of importance. The nlm movement l l applies some inertia, but the rotary shutter ita on shaft iii applies a relatively large inertia which tends to keep the movement shaft rotating at a constant speed in the event that the controlling connection formed by the tensed film between the movement shaft and the taire# up should for any reason be disrupted. That disruption of connection may occur by reason or temporary tightening slippage o the coil of film on 'che take-up spool, and is the more likely to be caused by slight uncontrollable variations oi? operation as the ultimate optimum design, with the minimum possible Vi'ilrn tension, is approached.

if we consider the mechanism without any inertia at all, or with low inertia values in proportion to the frictional motion resistance, values inherently involved; then on such nlm slippage the take-up spool will instantly go to a relatively high speed until the slippage is taken up. During that period in which the taire up runsaway"79 out of control, the movement speed suddenly and proportionately drops, causing a momentary hiatus in the proper exposure of the illm in the camera or projector. l

The shutter supplies an inertia in the mechanism to prevent or smooth out that hiatus. lts inertia prevents the movement from suddenly slowing down, and the take up from suddenly speeding up. The lm exposure is consequently kept more nearly uniform while the taire-up reel slowly increases speed to slowly take up the slippage with a minimum of iinal snap action on the lm.

in the event of lrn breakage from any cause, the lm movement will either come to a stop or to a relatively low speed if the break is between the movement and the take-up spool, or as soon as the break passes the movement. The take-up then f energies required to drive them; and as the energy required to operate the movement and all directly connected parts including the shutter and the lm feeds at normal speed is much greater than that required to rotate the free take-up at normal speed, the movement speed drops until those two energies are equal. In practice the movement either stops or comes to very low speed. That fact is immediately evident to the operator, and no excessive pile-up of the lm takes place between the movement and the take-up before he can stop the mechanism.

As has been indicated, the desired over-all gear ratio may be obtained by any suitable combination of the three gear ratios in the system shown in Fig. 3. The over-al1 ratio there is three-to-one; made up of a one-to-three ratio between the movement shaft and the ring gear, a three-to-one ratio in the diierential gearing between the ring gear and the sun gear, and a three-to-one ratio between the sun gear and the take-up. The iirst mentioned ratio, between the movement and the ring gear, is convenient because it allows the differential mechanism as a whole to operate at relatively low speed. ln the design as shown in Fig. 3, the relative decrease in that ratio from the one-to-one ration which would obtain if the movement shaft were driven directly from the ring gear, is oiset by having the ratios in the differential and between the sun-gear and take-up high enough to give an over-all ratio ci three-to-one.

Conversely, ior example, ii the gear ratio of ring-gear to sun-gear be decreased (e. g. sun gear made relatively larger) that decrease can be oiset by increasing either or both oi the other gear ratios. rlLhus the sun gear can in eect be made the same sizeas the ring gear (the differential becomes one of the automobile type) with a gear ratio o one-to-one. The desired over-all gear ratio may then be maintained by increasing either or both of the ratios between movement and ring gear, or between sun gear and taire-up.

Finally ll note that itis theoretically immaterial which or the three elements or the dirferentia, gearing is the one to be initially driven, and which ones drive the movement and the taire-up. Certain arrangements, such as the one shown here, may be more practical than others; but it is not necessary, for instance, that the planetary carrier be the initially driven element.

it can be shown that in any diiierential gearm ing, regardless or type, the velocities of the elements is expressed. by

P=radius or planetary asis S=pitch radius of sun gear R=pitch radius of ring gear Wpc=angular velocity of planetary carrier ltifa-:angular velocity of sun gear Wrrangular velocity of ring gear The expression (s) above holds true regardless of which is the driven member and regardless of what the other two members may drive. Within physical limitations, any conceivable relation between the three angular velocities and 'their associated tongues can be obtained within the eirguagua pression (4). And if for any reason, the relative values of S, R. and P do not in themselves give the desired `velocity and torque ratios between the movement and take-up, then those desired ratios can be reached by interposing lsuitable gearing external of the differential gearing and betweenits two driven members and the lm movement and take-up.

I claim:

1. In kinetographs, the combination of lm driving and exposing mechanism including a cyclic film feeding movement by which a predetermined feed-length of film is fed past an exposure point for each cycle of movement operation, a take-up spool on which the film is taken from the movement and wound in a coil of progressively changing diameter as the winding proceeds, the minimum peripheral length of coil on said spool being a large multiple of the cyclic feed-length of the movement, and driving means for said mechanism and the take-up spool comprising a differential gearing of three elements, initial driving means for one of the elements, and direct driving connections between the other twol elements and said mechanism and take-up spool, respectively, the over-all gearing ratio from said mechanism through the differential gearing to the take-up spool being substantially as high as is consistent with maintenence of take-up tension on the film by reaction from the driving torque of said mechanism.

2. The combination dened in claim 1 and in which the multiple is o f the order of sixteen, and the over-all gearing ratio is approximately three to one.

3. The combination defined in claim 1, and in which the film driving and exposing mechanism also includes a rotatingshutter cooperating with the movement, and in which said mechanism has a relatively high rotational inertia as compared 4o' with that of the take-up spool.

4. In kinetographs, the combination of lm driving and exposing mechanism including a lil movement by which the nlm is driven past an exposure point at relatively uniform average speed and including a rotating shutter cooperat` ing with the movement, said driving'and exposing mechanism having a substantial rotational inertia, a take-up spool on which the lm is taken from the movement and wound in a coil of progressively changing diameter as the winding proceeds, the rotational inertia of the take-up spool being relatively low as compared to that of said mechanism, and driving means for said mechanism, and the take-up spool comprising a differential gearing of three elements, initial driving means for one of the elements, and direct driving connections between the other two elements and said mechanism and take-up spool, respectively, the over-all gearing ratio from said mechanism through the differential gearing to the take-up spool being substantially as high as is consistent with maintenance of take-up tension on the illm by reaction from the driving torque of said mechanism.

5. The combination denedin claim 1 and in which the over-all gearing ratio is approximately three-to-one.

- GEORGE A. MITCHELL.

REFERENCES vCITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date v 1,235,249 Salsberg July 31, 1917 2,090,130 Kittel Aug. 17, 1937 2,147,776 Mitchell Feb. 21, 1939 2,196,358 Henisch Apr. 9, 1940 2,201,886 Dalotel May 21, 1940 FOREIGN PATENTS Number v Country Date 3,496 Great Britain Sept. 12, 1912 

